Guice Assisted Inject

assisted_inject

AssistedInject allows you to dynamically configure Factory for class instead of coding it by yourself. This is often useful when you have an object that has a dependencies that should be injected and some parameters that must be specified during creation of object.

Example from docummentaiton is a RealPayment

public class RealPayment implements Payment {
   @Inject
   public RealPayment(
      CreditService creditService,
      AuthService authService,
      @Assisted Date startDate,
      @Assisted Money amount) {
     ...
   }
 }
See that CreditService and AuthService should be injected by container but startDate and amount should be specified by a developer during the instance creation.

So instead of injecting a Payment you are injecting a PaymentFactory with parameters that are marked as @Assisted in RealPayment

public interface PaymentFactory {
    Payment create(Date startDate, Money amount);
}
And a factory should be binded

install(new FactoryModuleBuilder()
     .implement(Payment.class, RealPayment.class)
     .build(PaymentFactory.class));
Configured factory can be injected in your classes.

@Inject
PaymentFactory paymentFactory;

and used in your code

Payment payment = paymentFactory.create(today, price);